Lead frame and method of manufacturing the same

ABSTRACT

A lead frame includes a base material having a front surface for mounting of a semiconductor chip and a back surface for connection with an external board, and an Ni layer having a thick section and thin section. The thick section is formed on the back surface of the base material, whereas the thin section is formed on all or a part of the front surface of the base material. It is preferable that the thick section has a thickness ranging from 2.5 to 5 μm, and the thin section is 0.5-2 μm thinner than the thick section. The lead frame can be manufactured with improved productivity by forming an Ni layer on both front and back surfaces of the base material, and then etching only the Ni layer formed on the front surface of the base material.

BACKGROUND OT THE INVENTION

1. Field of the Invention

The present invention relates to a lead frame and its method ofmanufacture, and is particularly concerned with a lead frame used formanufacturing a semiconductor package with single-sided encapsulation,such as an SON (Small Outline Non-Leaded) or a QFN (Quad FlatNon-Leaded).

2. Description of the Related Art

There is increasing use of chip-size packages for semiconductor devicesmounted on a printed circuit board or the like due to market demandalong with downsizing and miniaturization of electronic devices. Inparticular, a semiconductor package with single-sided encapsulation suchas an SON package or a QFN package is commercially produced forsemiconductor device that uses a lead frame for its manufacture. Thesemiconductor package with single-sided encapsulation uses resin thatencapsulates or encloses a semiconductor chip or die placed on an uppersurface of the lead frame, while exposing a portion of the lead framealong a lower surface of the lead frame, i.e., a back surface of thepackage.

FIG. 8 is an example of such semiconductor device 9 with single-sidedencapsulation, which is manufactured by using a lead frame 5. The leadframe 5 is formed to have its pattern by etching or stamping (pressing)a base material 1 of a metal sheet. The patterned base material 1 isthen plated to have a three-layered plating film consisting of an Nilayer 2, a Pd layer 3 and an Au layer 4 over the entire surface of thebase material 1, i.e., a front surface 1 a, a back surface 1 b, and alateral surface 1 c.

Semiconductor chips 6 are then placed on respective die pad sections 5 aof the lead frame 5. Each semiconductor chip 6 is connected to the leadsections 5 b of the lead frame 5 by bonding wires 7. An upper surface(front surface 1 a) and a lateral surface 1 c of the lead frame 5 arecovered with resin 8 to encapsulate the semiconductor chips 6 mounted onthe front surface 1 a. This encapsulated unit is then cut apart orsingulated to produce multiple individual semiconductor devices 9.

The semiconductor device 9 with single-sided encapsulation has astructural feature whereby lower surfaces (back surface 1 b) of the leadsections 5 b are exposed from the resin 8 so that they come into contactwith an external board such as a printed circuit board (not shown). Thisfeature however suffers from insufficient adhesion between the leadsections 5 b and the resin 8, which leads to a problem in that some leadsections 5 b become detached from the resin 8 and falloff during thecutting step described above.

To solve this problem, Japanese Patent Application Laid-Open No.2006-93559 proposes a method of using two kinds of Ni-plating solutionsto form two kinds of Ni layers, each having different compositions onthe upper and lower surfaces of the lead frame. By forming these twokinds of Ni layers, a three-layered plating film having a rough surfaceand consisting of an Ni layer, a Pd layer and an Au layer is formed onthe upper surface of the lead frame. On the other hand, anotherthree-layered plating film having a smooth surface and consisting of anNi layer, a Pd layer and an Au layer is formed on the lower surface ofthe lead frame. This configuration improves adhesion between the resinand the lead frame.

To evaluate the adhesion between the resin and the lead frame having thethree-layered plating film with a rough surface, the adhesive strengthwas measured in the following manner. First, a three-layered platingfilm having the rough surface described above was formed on a metallicbase material. On this three-layered plating film, four resin moldingseach 2 mm in diameter were formed under a mold-filling pressure of 100kg/cm² and the molding conditions of 175° C.×90 seconds. These fourresin moldings were then hardened for eight hours at approximately 175°C. in an oven. The four resin samples evaluated were thus formed. Eachresin sample was then pushed sideways by applying an increasing load.The load value applied at the instant that the resin sample becamedetached was measured. Each load value thus obtained was divided by thearea of adhesion of the corresponding resin sample to determine the loadvalue per unit area. The average of these four converted valuesrepresents the adhesive strength between the resin and the lead frame.

This evaluation revealed that the adhesive strength between the resinand the metallic base material having the three-layered plating filmwith the rough surface was 19.9 MPa. For comparison, another metallicbase material was prepared on which a three-layered plating film havinga smooth surface and consisting of an Ni layer, a Pd layer and an Aulayer was formed. The adhesive strength of this conventionalthree-layered plating film was evaluated in a similar manner, and wasfound to be 9.5 MPa. These results show that the three-layered platingfilm having a rough surface improves adhesion and has greater adhesionthan the conventional three-layered plating film.

Japanese Patent Application Laid-Open No. 2006-310397 shows a techniquefor roughening the surface of a base material for the lead frame of acopper system. Specifically, this technique uses a micro-etchingsolution to slightly dissolve the metallic surface of the base material.In this manner, minute concavities and convexities are formed on themetallic surface of the base material.

On this roughened base material, a conventional three-layered platingfilm consisting of an Ni layer, a Pd layer and an Au layer was formed.Thereafter, the adhesive strength was evaluated in a manner similar tothat described above. The adhesive strength of this roughened basematerial with the conventional three-layered plating film was 11.8 MPa.It was thus confirmed that forming the three-layered plating film on theroughened base material also improves the adhesion and has greateradhesion than the conventional three-layered plating film formed on theconventional metallic base material, although its adhesive strength wasslightly inferior to that achieved by the technique shown in JapanesePatent Application Laid-Open No. 2006-93559 mentioned above.

The method of forming the Ni layers disclosed in Japanese PatentApplication Laid-Open No. 2006-93559 may however cause a problem ofwarping due to the stress difference between the Ni layers, which aresequentially formed on each of the front and back surfaces of the leadframe using two kinds of plating solutions. Moreover, use of the twokinds of plating solution needs longer plating apparatus and moreplating steps. Accordingly, the plating process requires complicatedmanagement and more processing time, which leads to low productivity.The micro-etching process shown in Japanese Patent Application Laid-OpenNo. 2006-310397 also has a problem that the etching process requiresadditional etching apparatus and more processing time, which also leadsto low productivity.

SUMMARY OF THE INVENTION

In view of these and other considerations, it is an object of thepresent invention to provide a lead frame having a plated layer toimprove adhesion between the lead frame and the encapsulation resinwithout reducing productivity.

To achieve the aforementioned objectives, a lead frame in accordancewith the present invention includes a base material having a frontsurface for mounting of a semiconductor chip and a back surface forconnection with an external board, and an Ni layer having thick and thinsections formed on the front and back surfaces of the base material. Thethick section is formed on the back surface of the base material,whereas the thin section is formed on all or a part of the front surfaceof the base material. It is preferable that the thick section has athickness ranging from 2.5 to 5 μm, and that the thin section is thinnerthan the thick section by 0.5-2 μm.

The lead frame according to the present invention may have the thicksection in a specified area of the front surface including a die-padsite and a wire-bonding site, and the thin section may be formed inother area than the die-pad site and the wire-bonding site.

The lead frame according to the present invention may have a Pd layerand an Au-plated layer over the thick section and the thin section. Thelead frame may include a die pad section and a lead section, and anoverhanging section having a reversed staircase shape may be formed onan edge of said die pad section and/or lead section. Either no Ni layermay be formed or an extremely thin Ni layer not more than 0.2 μm thickmay be formed on a lateral surface of the base material of the leadframe. On the lateral surface of the base material of the lead frame,the Pd layer and the Au layer may be formed directly or via theextremely thin Ni layer.

A method for manufacturing a lead frame according to the presentinvention includes a step of forming an Ni layer having a predeterminedthickness on a front surface of a metallic base material for mounting ofa semiconductor device and on a back surface of the metallic basematerial for connection with an external board, and a step of etchingonly the Ni layer formed on the front surface of the metallic basematerial such that the Ni layer formed on the front surface is thinnerthan that on the back surface. Specifically, the lead frame may beformed by forming an Ni layer having a thickness ranging from 2.5 to 5μm on front and back surfaces on a metallic base material, and byetching only the Ni layer on the front surface which corresponds to aside for mounting of the semiconductor device. In this manner, thethickness of the Ni layer on the front surface may be reduced to bethinner than the Ni layer on the back surface by 0.5-2 μm.

This manufacturing method has such feature that only one side of themetallic base material is processed by the etching solution before themetallic base material is formed to have a lead frame pattern. Thereforeit has minimal effect on the Ni layer on the opposite side of themetallic base material.

The method may further include a step of forming a predetermined maskover the Ni layer, a step of etching a part of the Ni layer exposed fromthe mask and a part of the metallic base material positioned below thepart of the Ni layer, a step of removing the mask, and a step of forminga Pd layer and an Au layer. The method may also include a step offorming an extremely thin Ni layer not more than 0.2 μm thick before thestep of forming the Pd layer and the Au layer. In addition, the methodmay include a step of forming a mask to create a thin section inspecified areas on the side for mounting of the semiconductor deviceafter forming the Ni layer with the predetermined thickness on both thefront and back surfaces of the metallic base material, and a step offorming the thin section by etching the Ni layer and removing the mask.

According to the present invention, a lead frame having improvedadhesion to the resin used for encapsulation can be formed withoutreducing productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a semiconductor device which ismanufactured by the use of a lead frame according to the firstembodiment of the present invention;

FIG. 2 shows a partially enlarged sectional view of one example of alead section of a lead frame according to the first embodiment of thepresent invention;

FIG. 3 shows a partially enlarged sectional view of another example of alead section of a lead frame according to the first embodiment of thepresent invention;

FIGS. 4A-4H show schematic flow diagrams illustrating the series ofsteps for manufacturing the lead frame of the first embodiment of thepresent invention;

FIG. 5 shows a cross-sectional view of a semiconductor device which ismanufactured by the use of a lead frame according to the secondembodiment of the present invention;

FIG. 6 shows a partially enlarged sectional view of one example of alead section of a lead frame according to the second embodiment of thepresent invention;

FIG. 7 shows a cross-sectional view of a left half of a semiconductordevice which is manufactured by the use of a lead frame according to thethird embodiment of the present invention; and

FIG. 8 shows a cross-sectional view of a semiconductor device which ismanufactured by the use of a conventional lead frame.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A lead frame according to the first embodiment of the present inventionis first described in reference to FIG. 1. A lead frame 10 of the firstembodiment includes a metallic base material 11 and an Ni layer formedon a front and a back surface of the metallic base material 11. The Nilayer is characterized by having a thin section 12 a and a thick section12 b. The thin section 12 a of the Ni layer is formed on all the frontsurface of the metallic base material 11. The front surface of themetallic base material 11 corresponds to a side for mounting of thesemiconductor device 6. The thick section 12 b of the Ni layer which isthicker than the thin section 12 a is formed on the back surface of themetallic base material 11. The back surface of the metallic basematerial 11 corresponds to a side for connection with an external board(not shown). It is preferable that the thick section 12 b has athickness ranging from approximately 2.5 to 5 μm, and that the thinsection 12 a is thinner than the thick section 12 b by approximately0.5-2 μm.

A layered structure 13 including a Pd layer 13 a and an Au layer 13 b isformed over the thin section 12 a and the thick section 12 b of the Nilayer, as shown in FIG. 2, which is a partially enlarged sectional viewof the lead frame 10. This layered structure 13 is also formed on thelateral surface of the metallic base material 11, which is substantiallyvertical to the front and back surfaces the of metallic base material 11of the lead frame 10. As indicated below, stamping or etching of themetallic base material 11 to form the lead frame pattern is performedafter forming the thin section 12 a and the thick section 12 b on themetallic base material 11. Accordingly, neither the thin section 12 anor thick section 12 b is formed on the lateral surface of the metallicbase material 11 of the lead frame 10. The layered structure 13 istherefore directly formed on the lateral surface of the metallic basematerial 11 of the lead frame 10.

As shown in FIG. 3, which is a partially enlarged sectional view of thelead frame 10, an extremely thin Ni layer 14 not more than approximately0.2 μm thick may be formed after etching or stamping of the metallicbase material 11. In this case, the extremely thin Ni layer 14 is formedover the entire surface of the lead frame 10, including the lateralsurface thereof. Therefore, the layered structure 13 is formed on thelateral surface of the metallic base material 11 of the lead frame 10via the extremely thin Ni layer 14.

As described below, the thin section 12 a is formed by consecutive stepsof forming a thick Ni layer on front and back surfaces of the metallicbase material 11 and etching only the Ni layer on the front surface formounting of the semiconductor chip. This method makes it possible toroughen a surface of the thin section 12 a that is to be attached to theencapsulation resin. This rough surface enhances its anchor effect,i.e., the encapsulation resin firmly adheres to the lead frame.Accordingly, it alleviates the aforementioned problem of the detachmentof lead sections from the encapsulation resin during the fabrication ofthe package. Note that the Ni layer to be exposed from the back surfaceof the package, which is formed on the back surface of the lead frame,is formed in the conventional manner. Therefore, the thickness andsurface smoothness of the Ni layer on the back surface remains the sameas the conventional packages with single-sided encapsulation.Accordingly, there is no resin leakage during the molding process forencapsulation. In addition, there is no deterioration of solderwettability that affects the connection between the semiconductorpackage and the external board.

A method of manufacturing the lead frame according to the firstembodiment of the present invention is hereinafter described inreference to FIGS. 4A-4H. First, as shown in FIG. 4A, the metallic basematerial 11 is prepared in the shape of a plate. Next, as shown in FIG.4B, Ni layer 12 having a thickness ranging from approximately 2.5 to 5μm is formed on both sides of the metallic base material 11. Then, asshown in FIG. 4C, one side of the metallic base material 11 is sprayedwith an etching solution to reduce the thickness of the Ni layer 12 onthis side by approximately 0.5-2 μm. In this manner, the thin section 12a is formed in a region that has been etched, whereas the thick section12 b is formed where there has been no etching.

Since the metallic base material 11 shown in FIG. 4C is not yetpatterned to have a lead frame pattern, the etching solution sprayed onone side of the metallic base material 11 has minimal effect on the Nilayer formed on the other side where the etching solution has not beensprayed. However, it is possible that some of the etching solutionspreads to the other side via the lateral surface of the metallic basematerial 11 and adheres to the Ni layer on the other side. Accordingly,as a precaution, the Ni layer on the other side may be protected by, forexample, a protective film. FIG. 4C shows such protective film 15, whichfully covers the other side of the metallic base material 11.

After the etching process, the protective film 15 is removed. Next, asshown in FIG. 4D, dry film resists 16 are laminated on both the thinsection 12 a and thick section 12 b of the Ni layer. Lithographicexposure is performed using a predetermined mask and then development isperformed. Consequently, as shown in FIG. 4E, resist patterns 16 a areformed on both sides of the metallic base material 11. Next, as shown inFIG. 4F, etching is performed to dissolve and remove the exposed regionsof the thin section 12 a and the thick section 12 b of the Ni layer,which are exposed from the resist patterns 16 a. Parts of the metallicbase material 11 located below these exposed regions are also dissolvedand removed by this etching process. Since the etching speed of themetallic base material 11 is much faster than that of the Ni layer, someparts of the Ni layer create burr-like or flash-like remainders. Toremove these remainders, it is preferable to perform Ni-selectiveetching. Thereafter, as shown in FIG. 4G, resist patterns 16 a areremoved. Finally, as shown in FIG. 4H, a layered structure 13 includinga Pd layer and an Au layer is formed over the entire surface of themetallic base material 11. In this manner, the lead frame 10 havingimproved adhesion to the resin is fabricated.

Note that, after removal of the resist patterns 16 a and before theformation of the layered structure 13 including the Pd layer and Aulayer over the entire surface of the metallic base material 11, theextremely thin Ni layer 14 not more than approximately 0.2 μm thick maybe formed over the entire surface of the metallic base material 11 asdescribed above in reference to FIG. 3. The extremely thin Ni layer 14can be formed on the lateral surface of the lead frame 10 where themetallic base material 11 is exposed. This extremely thin Ni layer 14has negligible effect on the adhesion of the thin section 12 a of the Nilayer formed by the etching.

It is preferable that thickness of the Ni layer 12 formed on both sidesof the metallic base material before etching should not be less than 2.5μm so as not to cause bending due to the difference in stress betweenthe Ni layer on one side and that on the other side. This bendingproblem may occur after the Ni layer 12 on one side has been etched toreduce the thickness by 0.5-2 μm. On the other hand, the thickness ofthe Ni layer 12 formed on both sides of the metallic base materialbefore etching should not be more than 5 μm. This is becauseconsiderable time may be required for the plating step, which isundesirable in commercial production.

An etching step to reduce the thickness of the Ni layer can take severalseconds for each 0.4-0.6 μm reduction, which means that the etching stepfor approximately 20 seconds can reduce the thickness of the Ni layer by2 μm or more. Therefore, the time required to reduce the thickness ofthe Ni layer by 0.5-2 μm is only a few tens of seconds, which does notsubstantially affect productivity.

Next, a lead frame according to the second embodiment of the presentinvention is described in reference to FIG. 5. The lead frame 20 of thesecond embodiment is similar to that of the first embodiment except thatthe Ni layer formed on the front surface has both thick and thinsections. Specifically, the lead frame 20 of the second embodiment ischaracterized in that the Ni layer formed on a side for mounting of asemiconductor device 6 has a thick section 22 b in areas A and B and athin section 22 a in an area other than the areas A and B. Note that thearea A refers to a die-pad site where the semiconductor device 6 isplaced, whereas the area B refers to a wire-bonding site where a wirebonding adheres. The thick section 22 b of the Ni layer is also formedon a back surface which is the side for connection with an externalboard (not shown).

This configuration makes it possible to provide the Ni layer, which ison the front side for mounting of the semiconductor device 6, with asmooth surface in the regions for mounting of the semiconductor device 6and for connection with the bonding wire 7. Accordingly, the Ni layer ofthe lead frame 20 not only provides a better electrical connection tothe semiconductor device 6 and the bonding wire 7, but also improvesadhesion between the resin 8 and the lead frame 20.

The lead frame 20 of the second embodiment may be formed as follows.First, in a manner similar to the first embodiment, an Ni layer having athickness ranging from approximately 2.5 to 5 μm thick is formed on bothsides of the metallic base material 11. Next, before spraying an etchingsolution over one side of the metallic base material 11, a mask having apredetermined pattern is applied over the Ni layer on the side to besprayed. This mask may be formed from a dry film resist. Thereafter, theetching solution is sprayed to etch the portion of the Ni layer exposedfrom the mask and to reduce the thickness thereof by approximately 0.5-2μm. The mask is then removed to obtain the configuration shown in apartially enlarged sectional view of FIG. 6. The structure shown in FIG.6 has a thick section 22 b that has a thickness ranging fromapproximately 2.5 to 5 μm and a thin section 22 a that is approximately0.5-2 μm thinner than the thick section 22 b.

After removing the mask, the steps shown in FIGS. 4D-4H are performed ina manner similar to the first embodiment, which provides a lead frame 20of the second embodiment. The lead frame 20 has such features that theNi layer on the side for mounting of the semiconductor device has athick section and a thin section. The thick section is formed in adie-pad site and a wire-bonding site, whereas the thin section is formedin an area other than the die-pad site and the wire-bonding site. Notethat the Ni layer on the side for the connection with the external boardhas a thick section which is similar to the lead frame 10 of the firstembodiment.

Next, a lead frame according to the third embodiment of the presentinvention is described. As shown in FIG. 7, the lead frame 30 of thethird embodiment is characterized by having an overhanging section Cthat has a reversed staircase shape formed on an edge of a die padsection 30 a and a lead section 30 b in addition to having an Ni layerwith a thick section on all or a part of a side for mounting of thesemiconductor chip, and a thin section on the other side for connectionwith the external board. Note that FIG. 7 shows one example of the leadframe 30 in which the thin section is formed on entire surface of theside for mounting of the semiconductor device 6.

With this configuration, the resin 8 can spread to the back surface ofthe lead frame 30 via the overhanging section C during resin moldingstep to encapsulate the semiconductor chip 6 mounted on the frontsurface. Accordingly, the die pad section 30 a and/or the lead section30 b can be much more firmly supported by the resin 8. Note thatalthough the overhanging section C is formed on both the die pad section30 a and the lead section 30 b in FIG. 7, the overhanging section C maybe formed on either the die pad section 30 a or the lead section 30 b.

The lead frame 30 of the third embodiment may be formed as follows.Before performing the step shown in FIG. 4D, the steps similar to thosedescribed in the first and second embodiments are performed to form anNi layer having a thick section and a thin section. Next, as shown inFIG. 4D, dry film resists 16 are lithographically exposed and developedin similar manner as the first and second embodiments except that twotypes of glass masks having different patterns are respectively used forthe front and back surfaces to create two different mask patterns on thefront and back surfaces.

These two different mask patterns make it possible to half-etch themetallic base material 11 only from the back surface. Namely, regions onthe Ni layer exposed from the masks and a part of the metallic basematerial 11 located below such exposed regions are etched to create theoverhanging section C having a reversed staircase shape. After removingthe masks, the step shown in FIG. 4H is performed. In this manner, leadframe 30 shown in FIG. 7 is fabricated.

EXAMPLES First Example

A metallic base material of copper sheet 0.2 mm thick and 180 mm widewas pretreated for plating. Thereafter, an Ni layer having a thicknessranging from 3.3 to 4.1 μm was formed on front and back surfaces of themetallic base material using a sulphamic acid Ni plating bath.

Next, one side (front surface side) of the metallic base material havingthe Ni layer was sprayed with an etching solution (ferric chloridesolution) for approximately 10 seconds to reduce the thickness of the Nilayer on this side by 0.8-1.0 μm. In this manner, a thin section havinga thickness ranging from 2.3 to 3.2 μm was formed.

Although no protective film using a resist was provided on the reverseside (back side) of the metallic base material, no substantial problemwas observed. In other words, an absence of protective film caused nosubstantial problem with regard to the spreading and adhesion of theetching solution to the Ni layer on the reverse side of the metallicbase material, provided that the spray etching duration did not exceedapproximately 20 seconds.

Next, dry film resists were laminated over the Ni layer formed on thefront and back surfaces of the metallic base material. Glass masks eachhaving a lead frame pattern were placed for lithographic exposure on thedry film resists. After lithographic exposure, the glass masks wereremoved and developed. In this manner, resist patterns were formed onboth sides of the metallic base material having the Ni layer.

Next, the metallic base material was etched to dissolve and remove apart of the Ni layer exposed from the resist patterns together with apart of the metallic base material located below the exposed part toform a lead frame pattern. The resist patterns were then removed. Notethat the Ni layer was formed on the front and back surfaces of themetallic base material having the lead frame pattern, but the Ni layerwas not formed on the lateral surface of the metallic base material.

A Pd layer of 0.10 μm thick was formed on all the surfaces (the upperand lower surfaces and the lateral surface) of the metallic basematerial having the lead frame pattern. In addition, an Au layer of 0.05μm thick was formed on the Pd layer. In this manner, the lead frame ofsample 1 was formed.

Samples 2-4 were also formed in a manner similar to sample 1, exceptthat the spraying times were changed. The characteristic feature ofsample 2 was that the Ni layer on one side was thinner than that on theother side by 0.4-0.6 μm. The characteristic feature of sample 3 wasthat the Ni layer on one side was thinner than that on the other side by1.3-1.6 μm. The characteristic feature of sample 4 was that the Ni layeron one side was thinner than that on the other side by 1.8-2.2 μm.

Samples 1-4 were evaluated in terms of the adhesive strength between theresin and the thin section of the Ni layer having the Pd layer and theAu layer in a manner similar to that described above. The adhesivestrengths of samples 1-4 were 23.4 MPa, 18.7 MPa, 23.2 MPa, and 21.3MPa, respectively.

From these results, it was confirmed that all samples 1-4 had sufficientadhesion to the resin. It was also confirmed that an etching process toreduce the Ni layer by 0.8-1.6 μm was particularly advantageous from theviewpoint of adhesive strength in comparison to the technique shown inJapanese Patent Application Laid-Open No. 2006-93559 using the rough Nilayer, which achieved the adhesive strength of 19.9 MPa mentioned above,and from the viewpoint of etching duration which affects productivity.

Second Example

In a manner similar to that in the first example, an Ni layer having athickness ranging from 3.0 to 3.5 μm was formed on the front and backsurfaces of the metallic base material using a sulphamic acid Ni platingbath. Next, a mask was formed on the side for mounting of asemiconductor device using a resist to protect a region for placing thesemiconductor device and a region for providing wire bonding. A regionof the Ni layer exposed from the mask was sprayed with an etchingsolution for approximately 10 seconds to reduce the thickness of theexposed region by approximately 1.0 μm. The mask was then removed.

Thereafter, the steps similar to those in the first example wereperformed to complete fabrication of a lead frame. The characteristic ofthis lead frame was that the Ni layer had a thick section in the regionfor placing a semiconductor device (die-pad site) and the region forproviding a wire bonding (wire-bonding site), and that the Ni layer hada thin section in a region other than the thick section. The Ni layeralso had a thick section on the reverse side for connection with anexternal board. A semiconductor device was placed on the die-pad site ofthe die pad section of the lead frame, and the semiconductor device wasconnected to the wire-bonding site on each of the lead sections of thelead frame via a bonding wire. Thereafter, the upper surface formounting of the semiconductor device and the lateral surface of the leadframe were encapsulated or enclosed with resin. After the cutting step,multiple semiconductor packages were obtained.

The lead frame obtained in this manner was provided with the Ni layerhaving similar thickness and smoothness to the conventional Ni layer notonly at the back surface of the lead frame but also in the region forplacing the semiconductor device and the region for providing the wirebonding. Accordingly, there was no substantial problem in placing thesemiconductor device or in providing the wire bonding. In addition, thethin section of the Ni layer that had been etched had better contactwith the resin, which improved adhesion to the resin as compared withthe conventional lead frame having the conventional Ni layer.

Although the present invention has been described in detail, thoseskilled in the art should understand that they may make various changes,substitutions and alterations without departing from the scope of theinvention in its broadest form. Thus, it is intended that the presentinvention covers the modifications and variations of this inventionprovided they come within the scope of the appended claims and theirequivalents. This application is based on Japanese patent applicationNo. 2008-250738, which is herein incorporated by reference.

1-16. (canceled)
 17. A method of manufacturing a lead frame comprisingthe steps of: forming an Ni layer on both front and back surfaces of ametallic base material to have the specified thickness; and etching onlythe Ni layer formed on the front surface which corresponds to a side formounting of a semiconductor device so that the Ni layer formed on thefront surface is thinner than the Ni layer formed on the back surfacewhich corresponds to a side for connection with an external board.
 18. Amethod of manufacturing a lead frame comprising the steps of: forming anNi layer having a thickness ranging from 2.5 to 5 μm on both front andback surfaces of a metallic base material; and etching only the Ni layeron the front surface which corresponds to a side for mounting of asemiconductor device such that the Ni layer formed on the front surfaceis 0.5-2 μm thinner than the Ni layer formed on the back surface whichcorresponds to a side for connection with an external board.
 19. Themethod of manufacturing a lead frame according to claim 18 furthercomprising: forming a predetermined mask over the Ni layer; etching partof the Ni layer exposed from the mask and part of the metallic basematerial positioned below the part of the Ni layer; removing the mask;and forming a Pd-plated layer and an Au-plated layer.
 20. The method formanufacturing a lead frame according to claim 19, wherein an extremelythin Ni layer not more than 0.2 μm thick is formed before the step offorming the Pd-plated layer and the Au-plated layer.